Inkjet printers include a drop ejection device and a supply of printing fluid such as ink for replenishing ink to the drop ejection device. In the case of thermal inkjet printing, the drop ejection device is typically referred to as a printhead. Printing is accomplished by the selective actuation of the printhead as the printhead is moved relative to a print media. One common type of previously used inkjet printer uses a replaceable print cartridge that contains a printhead and a supply of ink contained within the print cartridge. This type of print cartridge is not intended to be refillable. When the initial supply of ink is depleted, the print cartridge is disposed of, and a new print cartridge is installed.
Another type of inkjet printer makes use of an ink reservoir that is separately replaceable from the printhead. The replaceable reservoir can be positioned on a scanning carriage with the printhead or positioned off the scanning carriage. In the case where the ink cartridge is mounted off carriage, the ink cartridge is either continuously in fluid communication with the printhead such as being connected by a flexible conduit or intermittently connected by positioning the carriage at a refilling station. The use of a replaceable ink container allows for the replacement of the ink container separately from the printhead, allowing the printhead to be used until end of printhead life, reducing the cost per page of printing.
Regardless of the inkjet printer configuration, it is important that the system have an accurate means of indicating when a low or out of ink condition has occurred to avoid exhausting one or more of the ink supplies in the middle of a printing job. In the case of large format printing, the job or sheet must be scrapped and the job restarted resulting in waste. Moreover, it is important that the printing system stop printing when the ink container is nearly empty. Allowing the inkjet printhead to reach the state of complete ink exhaustion can result in operation of the thermal printhead without ink, which can result in catastrophic damage and failure of the printhead.
There are clear advantages to knowing when the ink container is out of ink as well as having the ability to detect ink levels at numerous positions on the ink container. For example, with large format printers, which use a considerable amount of ink for covering large printing surfaces, the ability to compare ink requirements with the amount of ink remaining in the ink container prior to printing would be invaluable. In addition, providing more comprehensive feedback to the user of ink use allows the user to better anticipate when the ink containers will require replacement.